Sliding of rolling elements in ball and roller bearings

In this article, there will be a breakdown of the different forms of “slippage” and some of the causes.

Figure 1 – Here we see the rolling / sliding areas in a ball bearing.
Graphic credit: SKF.

The term “slip” means that at a certain point on the rolling element / stroke contact there is relative movement. One surface has a different speed (speed and / or direction) than the other surface. An important component of a bearing that will make a significant difference in whether this slip causes damage or leaves no trace is the lubricant, and in particular the generation of an oil film between the two surfaces. This film of oil is the buffer between the two differently moving surfaces.

Normal rolling contact:
With the rolling elements, there is always a slip on the contact ellipse. In this image, the yellow triangles are where there is rolling contact, and all other areas are sliding contact.

Figure 2
Graphic credit: SKF.

Thus, even in a normally operating bearing, there is slippage, hence the importance of a lubricating film to separate the rolling surfaces.

Abnormal contact:
Abnormal contact results from these cases:

Three points of contact:
With a QJ (4 point angular contact ball bearing) bearing, the bearing is internally designed to make contact in either of two planes, however, there should only be one plane. in contact at some point. This allows the bearing to take a thrust load in both directions, saving space compared to a pair of single row angular contact ball bearings or double row ball bearing.

If a radial load is applied to this bearing, the ball will contact the outer race in two places, while still only contacting the inner race in one place. The ball will rotate in plane with the outer and inner ring contacts 180 degrees to each other, and the second contact on the outer ring will slide. For this reason, these bearings are mainly installed with a relaxed housing seat so that the outer diameter of the outer ring does not contact the housing bore, thus preventing unintentional radial load from being applied to the bearing.

figure 3
Graphic credit: SKF.

Two-point contact on different planes
With a pair of angular contact bearings, there is one bearing that takes the thrust load and the other bearing is unloaded or shares some of the radial load. With lower speeds, the unloaded bearing will contact the races in two places, and the ball will spin on a single plane with rolling contact with both races.

In higher speed applications, a centripetal force will act on the ball to move it in a radial direction. When this happens, the ball will contact the outer ring on a different plane than the inner ring and one of those points of contact, usually with the outer ring, will slip.

Figure 4
Graphic credit: SKF.

Rolling element speed / moment of inertia
In a radially loaded bearing, the rolling element moves around the bearing and only in full contact with the two rings in the load area. When in the load zone, contact with the inner and outer rings provides traction such that the roller is driven and rotates.

When the roller is outside the load zone, it is no longer driven by the stroke contact. He is pushed by the cage through the unloaded area. As it is pushed, the frictional contact with the cage bar will slow the speed of rotation of the rolling element. In addition, any frictional effect of grease or oil can also contribute to slowing down the rotation of the roller. Depending on the size of the bearing, there may be sufficient distance across the load zone that the relative speed of the rolling element surface and the track surface becomes significantly different.

When the roller then enters the loading area, it quickly accelerates to racing speed. Often times with larger rollers there will be some distance the roller needs to reach speed to match the running surface speed because the larger roller mass has a greater moment of inertia. This is the short distance that slips, but often there is wear of the adhesive.

In fact, this type of slippage occurs in other cases. The basis is that the rolls undergo cyclic loading and unloading. This can also occur with double row bearings with opposite contact angles (such as a spherical roller bearing) in which the axial load oscillates, causing cyclic loading and unloading of the rolling element assembly.

One example I encountered was a spherical roller bearing in a newsprint machine. A new felt was installed in the press section, and due to certain properties of this felt, it introduced a rocking motion in one of the suction rollers. The sway of the roller could be observed visually. As for the bearings inside the roller, there was a spherical roller bearing which underwent cyclic axial loading, in which one stroke was loaded axially and then the other at a high frequency (i.e. i.e. a number of cycles per minute). With each cycle there was a landing and roller heave, and since it was a large bearing with massive rollers, on each landing there was a skid event which caused early bearing failures. .

Figure 5
Graphic credit: SKF.

Directional differences
In this case, the situation arises with spherical roller bearings in an application, where the outer ring rotates, the inner ring is fixed, and there is angular misalignment between the inner and outer rings.

Traditionally, the rollers of spherical roller bearings have been guided by the rotating inner ring, and due to the nature of the spherical bore of the fixed outer ring, the rollers always have a corresponding curved surface on which to roll.

However, when the rotation of the rings is reversed and the outer ring rotates and the inner ring is stationary, the track of the rollers on the outer ring has a different vector than the rotation of the outer ring. There is a slight angular difference which increases as the misalignment increases. While it might be hard to imagine, we can all see a similar case when a transport truck takes a turn and we observe the rear wheels.

Acceleration of rotating shaft
A very common application for skidding is electric motors with cylindrical roller bearings, and most often when these motors are tested in the rewinding workshop. If there is no radial load applied to the drive end of the motor during the test, there is a high risk of the rollers slipping. Other factors may include the speed at which the engine is started. When starting at “full speed” immediate acceleration of the engine may cause slippage, accelerating revs is best practice to avoid the possibility of skidding. MRO
Douglas Martin is a heavy machinery engineer based in Vancouver. He specializes in the design of rotating equipment, failure analysis and lubrication. Contact him at [email protected]

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